Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition, overlay and other parameters of nanoscale structures.
Ongoing reductions in feature size, increasing geometric complexity, and more diverse material compositions of semiconductor devices impose difficult requirements on optical metrology systems that are relied upon for process development and process monitoring. To achieve device performance requirements, the thickness and composition of the thin films (e.g., oxide, nitride, metal layers, etc.) formed on a silicon wafer must be accurately controlled during the semiconductor device manufacturing process.
Single Wavelength Ellipsometry (SWE) based measurement techniques and systems are often employed to measure thin film characteristics. SWE systems utilize model-based measurement techniques to determine physical properties of film structures based on polarization properties of light reflected from the structure under measurement. Exemplary metrology systems and techniques are described in detail in U.S. Pat. No. 6,734,968 issued on May 11, 2004, U.S. Pat. No. 7,006,222 issued on Feb. 28, 2006, and U.S. Pat. No. 7,253,901 issued on Aug. 7, 2007, all assigned to KLA-Tencor Corporation, the contents of each are incorporated herein by reference in their entirety.
In many advanced film measurement applications, SWE systems are preferred for their excellent measurement repeatability and optical stability. The single wavelength polarized light source delivers constant light output and excellent wavelength stability. SWE systems exhibit excellent tool-to-tool matching performance among multiple tools in the same, or different, fabrication facilities. This enables sharing of ellipsometry models, measurement recipes, and optical constants across multiple SWE systems.
However, the light sources employed by SWE systems exhibit large coherence lengths, for example, on the order of tens of meters. This leads to significant coherence artifacts in measurement signals that can be detrimental to system performance. Coherence based artifacts arise in many different circumstances. In one example, light diffracted from the edges of a metrology target leads to interference along the propagation path between light reflected from inside the boundary of the metrology target and light reflected from outside the metrology target. In another example, ghost images arise due to interference between even numbered reflections from surfaces of optical elements. In another example, measured data is contaminated by scattering effects from optical surface roughness and coatings, particulate contaminants, black surface treatments, and other light interactions with opto-mechanical structures.
Contaminated light does not exclusively carry information about the measurement target box. Any amount of contaminated light detected by the SWE system contributes to measurement error. The minimum target size that can be measured within a given thickness measurement error tolerance is often termed the “spot size.” The measurement spot size is a function of detected, contaminated light. The larger the amount of detected contaminated light the larger “spot size” will be measured for a given thickness error criteria. In some examples, contamination light levels must be less than 10−5 of the detected light to meet the measurement error specification for a reasonable spot size. As spot size requirements and measurement error requirements continue to grow more stringent, further reductions in contamination light are needed.
Future metrology applications present challenges due to small feature size and multi-parameter correlation. Improvements to SWE systems are desired.